1. Field of the Invention
This invention relates generally to fluid filters, and more particularly is directed to a fluid filter device especially suited for installation in an automotive power steering system.
2. Background of the Invention
Many modern automotive vehicles are provided with power steering systems. In such power steering systems, a power steering pump driven by the engine of the vehicle is connected through pressure and return lines or hoses with a power steering gear for circulating hydraulic fluid under pressure through the latter, and such power steering gear has valves controllable in response to manipulation of the steering wheel for directing the flow of fluid under pressure so as to provide a substantial portion of the force required for the steering of the vehicle. Since the valves and other mechanisms of the power steering gear are manufactured with very close tolerances and small clearances in order to properly perform their respective functions, the presence of metal, sand or other small particles, such as are detached from the interior surfaces of the pressure and return hoses when the latter deteriorate, in the hydraulic fluid being circulated can cause serious damage to the power steering gear and/or pump, and more particularly can cause the valves of the power steering gear to lock or stick briefly in positions other than those required for proper steering of the vehicle. It has been found that many serious automobile accidents resulting from swerving of an automobile out of its designated lane have been caused by power steering failures attributed to metal, sand or other particles or contaminants contained in the circulated hydraulic fluid of the power steering system and producing the described locking, sticking or even "self-steering" thereof.
In many existing power steering systems, the only protection against the above described hazard is in the form of a filter constituted by a single disk of metal mesh or screening extending across the interior of a casing which is usually interposed in the return line or hose. Apart from the inadequacy of such disk of metal mesh or screening in achieving full filtration of all particles and contaminants from the circulated hydraulic fluid, it will be apparent that, when the disk of metal mesh or screening becomes clogged with particles that have been filtered from the hydraulic fluid, the circulation of fluid between the power steering pump and the power steering gear is positively blocked with the result that operation of the power steering system is arrested without prior warning to the driver at a time when the power assist for steering may be most urgently required.
Although not particularly intended for use in automotive power steering systems, filter devices have been provided, for example, as disclosed in U.S. Pat. Nos. 2,389,814 and 3,334,747, in which a tubular filter element is disposed within a cylindrical casing having inlet and outlet passages at its opposite ends so that the fluid is normally passed radially through the wall of the filter element, and further in which a bypass valve is normally closed by a spring but is opened in response to the increase in the pressure differential between the inlet and outlet passages when the filter element is clogged so as to bypass the latter and ensure a continued flow of fluid through the device. However, in such existing filter devices which bypass the filter element when the latter is clogged, the bypass flow of fluid is unfiltered. Thus, if such a filter device was installed in a power steering sytem, the clogging of the filter element, for example, as a result of serious contamination of the hydraulic fluid with foreign particles, would cause such contaminated hydraulic fluid to be circulated, without any filtration thereof, through the power steering gear following the opening of the bypass valve so as to again create the hazard of a catastrophic malfunction of the power steering gear. Moreover, in the existing filter devices provided with a bypass valve, as described above, the flow rate past the valve, when the latter is opened in response to clogging of the filter element, is not appreciably different from the normal flow rate through the filter element so that, if such a filter device is installed in an automotive power steering system, the opening of the bypass valve in response to clogging of the filter element is not indicated to the driver as a detectable decrease in the power assist of the steering system. Further, in the existing filter devices provided with a bypass valve, the surface area against which the pressure differential acts for opening the valve is not appreciably greater when the valve is in its opened condition than when the valve is closed with the result that, following clogging of the filter element, there is a tendency for the valve to fluctuate between its fully opened position and an at least partially closed position to produce a corresponding undesirable fluctuating rate of flow of the fluid through the power steering gear.